2011 Theses Doctoral

Interfacial Studies of Organic Field-Effect Transistors

Jia, Zhang

Organic field-effect transistors (OFETs) are potential components for large-area electronics because of their attractive advantages: light weight, cost-effective and large-area processability, flexibility and resonable performance potential. However, the commercialization of OFETs faces several technical obstacles. Low mobility of organic semiconductors limits the current-carrying capacity; high operation voltage restricts their use in many applications; easy degradation in air and instability under electrical stress usually make the lifetime too short to be useful; and contact resistance and contact matching also limit the charge injection to the semiconductor. Many of the above problems relate to interfaces in OFETs. There are two important interfaces in OFETs. The interface between organic semiconductor and the dielectric layer is of crucial importance since it is the location where charge transport in the channel occurs. The other important interface in OFETs is between the semiconductor and the contacts, where the charge injection and removal happen during device operation. Surface treatment of the contacts for bottom-contact devices is usually necessary to achieve both a good semiconductor microstructure and excellent contact performance. Great effort has been applied to improving device performance, primarily by focusing on enhancing device mobility to increase current capacity and improving subthreshold behavior to reduce the operation voltage. One approach to improving both figures of merit is to use a high-capacitance gate dielectric, which reduces the operating voltage and increases the mobile charge carrier density for a given gate voltage. Operating at a higher channel charge density improves the effective mobility in OFETs. I first demonstrate the use of nanoscale high-$kappa$ materials based on barium titanate (BT) which are normally ferroelectric as gate dielectrics where their high dielectric constant is desirable but ferroelectric hysteresis is not. Self-assembled monolayer (SAM) treatment of the dielectric has been used to improve the morphology of subsequent deposition of organic semiconductor. The dipoles within the SAM, however, dramatically change the electrical performance in terms of threshold voltage and mobility. This thesis reviews the SAM treatment and explains why there is a substantial change in threshold voltage. During the fabrication, reactive agents can also reside at the interface between the semiconductor and the dielectric layer. Their chemical and structural effects are minor but their effect on electrical performance can be significant. This problem is studied using spectral photocurrent and $1/f$ noise measurement by comparing OFETs whose polymer gate dielectric is exposed to UV ozone prior to semiconductor deposition with control OFETs whose semiconductor/dielectric interface is produced in a nearly oxygen-free environment. Both of the techniques have shown that the interfacial trapping sites created by oxygen treatment play an important role in electrical performance. One approach developed to improve the performance of bottom contact source/drain electrodes is to treat the contacts with thiols before deposition of the semiconductor. Especially suggestive evidence shows that thiols that increase the effective work function of the contacts (textsl{e.g.} fluorinatedthiols) yield better device performance than work function decreasing thiols (textsl{e.g.} alkane thiols). We compare two technologically relevant thiol treatments, an alkane thiol (1-hexadecanethiol), and a fluorinated thiol (pentafluorobenzenethiol), in pentacene organic field effect transistors. Using textit{in-situ} semiconductor deposition, X-ray photoemission, and X-ray absorption spectroscopy, we were able to directly observe the interaction between the semiconductor and the thiol-treated gold layers. Our spectroscopic analysis suggests that there is not a site-specific chemical reaction between the pentacene and the thiol molecules. A homogeneous standing-up pentacene orientation was observed in both treated substrates, consistent with the morphological improvement expected from thiol treatment in both samples.Our study shows that both the HOMO-Fermi level offset and C $1s$ binding energy are shifted in the two thiol systems, which can be explained by varied dipole direction within the two thiols, causing a change in surface potential. The additional improvement of the electrical performance in the pentafluorobenzenethiol case is originated by a reduced hole injection barrier that is also associated with an increase of the density of states in the LUMO. In OFETs, the accumulated charges are not evenly distributed along the channel especially when the OFETs are operated in the saturation region, where the drain side has much fewer accumulated charges than the source side due to the cancellation of effective gate voltage by the drain voltage. Thus the carriers should be less mobile on the drain side where the trap states are filled less adequately, and one should expect a varied mobility across the channel. For the same reason, the saturation current formula $I_{DS} = frac{W}{2L}mu C_{i}(V_{GS}-V_{th})^{2}$ for silicon MOSFETs is not suitable for OFETs, and the mobility calculation based on linear fitting of $sqrt{I_{DS}}$ to $V_{GS}$ is problematic. In the last part of this thesis, I have reviewed the curve fitting method and quasi-static capacitance-voltage (QSCV) method for deriving linear mobility in OFETs. Further, we have measured spatially resolved photocurrent in OFETs operated in the linear and saturation regions. Because the photogenerated charge is constant as a function of bias, spatially resolved photocurrent measurement locally measures the product of channel field and mobility. This product can be used to derive the local mobility across the channel. Our results directly show that in the saturation region, mobility decreases from source contact to drain contact due to the decreased density of carriers on the drain side.